Time delay means for centrifugal switch



May 24, 1960 E. BURRELL 2,938,088

TIME DELAY MEANS FOR CENTRIFUGAL SWITCH Filed June 2, 195o ELL/5' EUHELL INVENTOR BY MCSW' ATTORNEY` 'United Staffa Patent; 0

2,938,088 TIME DELAY MEANS FOR CEN'mlEUGAL swrrcrr Ellis Burrell, Rochester, N.Y., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed June 2, 1950, Ser. No. 165,976

8 Claims. (Cl. 200-80) with two electrical fcontact members insulated from each other and electrically communicating with the interior -ofthe compartment. -Usuallyone contact memberis the tubular shell and a second contact member is mounted -in one end of the shell and insulated from it. The conductive fluid normally completes the electrical circuit between the contact members. The restriction partition ,between the two compartments is normally provided with restricted passages throughwhich the mercury is caused to flow from one compartment into the other by the pressure jof centrifugal force resulting from spinning of the mercury switch. The electrical circuit between the two contact- `members is broken when a portion of the conductive u-id has fiowed through the restricted passages lto,uncover the end of the second contact member and :open the conduction path through the mercury. The time `requiredto break the electrical circuit between the two Contact members after rotation begins is dependent upon .the volume of mercury that must flow to uncover the end yface of the second contact member and the rate of -ow of the mercury through the restricted passages, the latter factor depending upon the pressure of the fluid against'the restriction partition due to centrifugal force 'and lthe nature' of the restricted passages through the restriction partition.

Thev present invention relates particularly to the means 'for' controlling the rate of ow of the conductive uid from one compartment into the other.

- Heretofore, the restriction partition which divides the tubular shell into two compartments was constmcted of -fibrous material or of porous sintered metal. Fibrous -material was unsuitable to withstand the relatively high pressure developed by highl speed rotation, since the initial blow of the mercury against the restriction partition often caused the fibrous material to give way. Furthermore, when ibrous material `was utilized, it was difcult to reproduce units 4having identical time delays in the mass production of mercury switches, The introduction of sintered metal in place of the iibrous material did increase the ability of the restriction partition to withstand high pressures. However, i't'is extremely difcult to control the uniformity of porosity` of the sintered metal in production on a quantity basis, and a one hundred percent` inspection of the restriction partitions for time delay isnecessary. Furthermore, 4the sintered metal timing unitis expensive to manufacture, and the' diiculty of'reproducingv uniformity' of porosityoften results in rejections as high as twenty-five percent after testing of the timing units. If the powdered metal particles from which the sintered metal partition is formed are not unil form in size, small particles wedge in and ll the pores and voids between particles of the sintered metal with the result that the timing of the mercury switches is erratic.

It has also been found that, after a storage period of several months, a surface amalgamation occurs between the mercury and the sintered metal which plugs the pores and voids in the sintered nickel or other metal restriction partition and prevents the flow of mercury therethrough unless extremely high pressure is utilized to start the flow of the conductive fluid. This ,surface amalgamation results in a high percentage of the switches becoming inoperable in a few months after their manufacture.

It is an object of the invention to provide, in a switch of the character described, time delay means whereby the interval required for the passage of a given volume of mercury under rated spin conditions may be accurately controlled. It is a further o-bject of the present invention to provide such time delay means which can be produced on a quantity basis at considerably lower cost than sintered metal time delay elements.

It is also an object of the invention to provide, in a switch of the character described, time delay means sufticiently accurate in allowing the mercury to flow between the two compartments under given time delay characteristics that the one hundred percent inspection is eliminated and only spot checking of the timing interval is necessary. l

A still Afurther object o=f the invention is to provide, in a switch of the character described, time delay means which has'an extremely low tendency to amalgamate with the mercury and will thusmaintain a constant resistance to the ow of mercury therethrough regardless of the length of time the switches are stored before use.

A still further purpose of the invention is to provide, in a switch of the character described, improved means of insulating the central contact assembly from the tubular shell and of sealing the open end of the shell which will prevent leakage of mercury from the initially filled compartment over the maximum range of temperature to which a projectile containing the switch is subjected.

In accordance with the essential feature of the invent-ion, these and other objects are attained by forcing the mercury under centrifugal pressure to flow through a restricted foraminous orifice formed between Itwo abutting flat surfaces roughened by Sandblasting or etching one or both of the surfaces. The roughened surfaces are held in tight contact, and the mercury is retarded in owing through the eld of minute craters formed betweenthe surfaces.

Other objects and advantages of the invention will become apparent from an examination of the drawings and accompanying description wherein a preferred form of the invention is shown and described.

In the drawings:

Fig. 1 is a longitudinal sectional view through a mercury switch assembly embodying the invention;

Fig. 2 is an end view of the construction shown in Fig. l;

Fig. 3 is a bottom view of -the top member of a pair of roughened. disks which comprise the restriction partition in the mercury switch shown in Fig. l;

Fig. 4 is the top view of a second'roughened disk which complements the disk shown in Fig. 3 to formthe restriction partition utilized in the mercury switch of Fig. l;

Fig. 5 is a longitudinal sectional view through a switch of slightly/:different form embodying the invention;

Fig. 6 is a top view of a roughened disk included in the restriction partition of the switch shown in Fig. and

Fig. 7 is apartial longitudinal view through a form of the invention slightly modified from the embodiment 'shown in Fig. 5 to provide a switch that is particularly adapted for mass production.

Fig. 1 illustrates a two-compartment delay switch in which mercury 2.0 contained in one compartment normally completes an electrical circuit between two electrical contacts and is ymoved by centrifugal force through a restricted passage into the other compartment 30 to open the electrical circuit between the two contacts. A tubular shell 11, preferably of deep drawing steel, is restricted in diameter at one end 12 to provide a shoulder 13 which supports a stainless steel tubular sleeve 14 thereon which may alternatively be constructed of copper, Monel metal or any other suitable conductive material. The sleeve 14 is counterbored at the lower end thereof to form a larger diameter section 1S into which two stainless steel disks 16 and 17 are assembled, and the edge of the sleeve 14 is vspun over at the lower end thereof to seal the disks 16 and 17 -within the larger diameter section 15. As will be described later, at least one of the adjacent faces of the disks 16 and 17 is roughened by Sandblasting or etching to form a field of minute craters between the abutting 'surfaces ythrough which mercury is caused to flow to provide the desired time delay.

The shell 11 is adapted to form one terminal of the switch, and a connection lug 19 is spot welded or otherwise secured to the outer periphery of the tubular shell 11 near the open end thereof. Approximately one half of the thickness of the shell 11 at the open end thereof is 'sheared downward to form an annular protuberance 18 which holds the sleeve 14 firmly against the shoulder 13. A central contact assembly including a central rod 21 insulated from a metallic flanged ring 22 by a glass bead 23 is seated against the annular protuberance 18, and the edge of the shell 11 at the open end thereof is crimped to clamp the flanged ring 22 against the annular protuberance 18.

In mercury switches heretofore constructed, polystyrene or other insulating material disks were provided to isolate the central contact assembly electrically from the tubular 'shell 11, and the open end of the shell 11 was spun over the insulating disks to seal the shell 11 against leakage 'of mercury. It has heretofore been impossible to provide va satisfactory metal-to-metal seal at the open end of the shell 11 by positioning a metal disk against the top 'of the tubular sleeve 14 and spinning over the open end of the shell 11 against the metal disk. Expansion or contraction of the shell 11 relative to the tubular sleeve 14 caused sufficient play between the top of the tubular sleeve 14 and the spun-over portion of the shell 11 to make it impossible over a range of temperatures to utilize a metal disk at the open end of the shell toprevent leakage of mercury out of the compartment initially filled with mercury. This diliiculty has been overcome in the preferred embodiment of the invention shown in the drawings by the combination of the shearing operation hereinbefore described to form the annular protuberance 18 and by spinning or setting an annular groove or set 24 in the spun-over portion of the shell 11 to pinch the flanged ring 22 lirmly between the annular protuberance 18 and this spun-over portion. This setting operation etfectively squeezes the anged ring 22 between two raised Iportions on the same metal member, i.e., the shell 11, so

that the seal is unaffected by expansion or contraction of the shell 11 relative to the tubular sleeve 14, and Va 1eak lproof metal-to-metal seal is thus effected 'at the open end ofthe shell, which prevents any leakage o'f mercury out iof the shell 11 regardless of the range of temperatures to which the switches Vare subjected.

I n one application of a centrifugal mercury switch, the shell '11 is mounted in an electric fuze with the end 12 away from the axis of rotation of an artillery projectile. It has been found that polystyrene and rubber-faced phenolic resin washers, used heretofore to insulate the central contact assembly from the tubular shell 11, are not satisfactory under certain climatic conditions and under the high temperatures and pressures to which `a projectile is subjected in tiring. At extremely low temperatures the washers shrink so much they Adestroy the seal at the central contact assembly and allow leakage of mercury, while at extremely high temperatures softening and bucklink of the washers destroy the seal. 1n the embodiment of the invention illustrated in Fig. 1, an approximately cylindrical glass bead 23 insulates the central rod 21 from the flanged ring Z2, both of which are formed from an alloy consisting of approximately 54% iron, 28% nickel, and 18% cobalt and marketed under the trade name Kovar." This alloy has an exceedingly low coeicient of expansion which is suliiciently close to that of glass so that no cracking or breakage ofthe glass bead 23 will occur over the maximum temperature range to which a projectile is subjected. The Kovar alloy of the central rod 21 and the flanged ring 22, as well as the stainless steel of the sleeve 14 and the disks 16 and 17, have an extremely low tendency to amalgamate with mercury and prevent contamination and the formation of a sludge within the compartment normally filled with mercury. Furthermore, the use of metals having a low tendency to amalgamate with the mercury prevents the deposition of a lm `of an oxide of mercury across the vswitch which, in switches heretofore constructed, often caused a high resistance short circuit between the two electrical contacts after n suicient volume of conductive uid had flowed through the restriction partition to open the conduction path through the mercury. In addition, the use of the stainless steel disks 16 and 17 in the restriction partition pre vents the surface amalgamation" heretofore encountered in switches manufactured with sintered metal restriction partitions. This surface amalgamation between 'the vmercury and the surface of sintered metal restriction partitions plugs the pores and voids in the sintered metal after the switches have been stored for a few months and thus prevents the ow of mercury through the restriction 'partition unless an exceptionally high pressure is developed to -initiate the flow of the conductive fluid. The use of roughened disks 16 and 17 of stainless steel, which has an extremely low tendency to amalgamate with the mercury,

eliminates 'any possibility of surface amalgamation at lthe restriction partition regardless of the length of time the switches are stored before use. A terminal 28 issecured to the central contract assembly by spinning over the outer end of the central rod 21.

The roughened stainless steel disks i6 and 17 divide .the shell 11 into two compartments, one of which, formed mainly in the tubular sleeve 14, is normally filled with the conductive fluid. The second compartment or sump 30, formed in the restricted diameter end 12 of the shell 11, acts as a receiver for the mercury. At least one of the adjacent faces 31 and 32 of the disks 16 and 17 respectively is roughened by Sandblasting or etching to form a field of minute craters in the interstices between these abutting surfaces. A plurality of holes 34 drilled in the top surface of the disk -16 along the circumference of a circle yconcentric with the disk 16 open into an an nular groove 35 formed in the lower face of the disk 16. A central hole 3-6 is provided axially through the lower disk 17. -During the liight of a projectile, centrifugal force causes mercury to flow .through the holes 34 into the annular groove 3S and thence radially between the surfaces 31 and 32 to escape through the central hole.36 yinto the sump 30. The resistance to the flow of mercury through the vrestricted foraminous 'orifice vformed bctween the surfaces 31 `and 32 is such that mercury will `not escape into the sump 3l) at atmospheric pressure d ue to jolts'and tumbling that the switch normally undergoes in handling. The coarseness of the roughened 'surfaces 31 and 32, the pressure under which these surfaces are held in contact, and the pressure developed by centrifugal force determine the rate at which the mercury will escape into the sump 30. As the angular velocity which a given type of projectile attains is approximately constant from round to round', the rate at which the mercury escapes from the initially filled compartment can be varied by providing roughened surfaces which have been sandblasted or etched to different degrees of coarseness. If sandblasting is utilized, the roughness of the surfaces can be accurately controlled over a 'wide range by varying the size of the grit in the Sandblasting stream. To assure that the desired roughness is reproduced on each disk 16 and 17, the surfaces 31 and 32 are overblasted, i.e., subjected to the stream of grit for a longer time than necessary to attain the desired coarseness.' This overblasting effects a saturated condition of the surfaces which is constant from piece to piece, further blasting having no effect except to -wear away metal, leaving the surfaces with the desired roughness. The time required for a given volume of mercury to ow into the sump 30 to uncover the end face of the central contact can thus be accurately regulated by providing disks 16 and 17 with faces roughened by Sandblasting or etching to the required degree of coarseness, and time delays ranging from 0.1 second to 3.0 seconds under a given prsure head have been obtained.

The sintered metal restriction partition utilized heretofore in centrifugal switches was difficult to manufacture with controlled porosity on a quantity basis. Timing of mercury switches using such sintered metal restriction partitions was erratic and necessitated one hundred percent testing of the time delay required to pass a given volume of mercury under a given pressure head. Besides being expensive to manufacture originally, the inspection required on time delay often resulted in rejection of the sintered metal restriction partitions as high as twenty tive percent. The roughened disks of the preferred embodiment of the invention are comparatively simple to manufacture, and' the coarseness of the roughened surfaces can be accurately controlled with the result that the timing of the mercury switches is stable regardless of the length of time the switches are stored. The delay in forcing the mercury between the roughened surfaces is suiciently 4constant from switch to switch that the one hundred percent testing of time delay is eliminated and only sample or spot testing is required.

The modification of the invention shown in Fig. 5 completely eliminates any possibility of leakage of the conductive lluid around the restriction partition. In this form of the invention a single roughened disk is held in close contact with the flat bottom surface of a stainless steel cup which initially contains the mercury. The conductive uid is forced by centrifugal pressure through a central aperture in the bottom of the cup and thence radially outward through the restricted foraminous 'orifice between the single roughened surface and the tiat bottom surface of the cup.` A single roughened' stainless steel disk 51 is shown supported on the shoulder 13 in the tubular shell 11.' As shown in Fig. 6, a plurality of circumferential indentations 52 which extend farther toward the axis of the shell 11 than the annular shoulder 13 are provided in disk 51, and the upper surface 57 thereof is roughened by Sandblasting or etching as described hereinbefore. Astainless steel cup 53 is assembled against the disk 5-1, and approximately one-half of the thickness of the shell 11 at the open end thereof is sheared downward to form an annular protuberance 18. The pressure of the shearing operation seats the cup 53 firmly against the disk 51 to hold the flat bottom surface 56 thereof in close contact with the roughened surface 57 and to form a leakproof seal between the cup 53 and' the shell 11 in the same manner that a leakproof seal was formed between the sleeve 14 and the shell 11 in the modification shown irrFig.V 1. The cup 53 andthe disk 51 divide the shell 11 into two compartments, one of which, formed lnaiiily in the cup 53, is normally illed with the conductive uid. The stainless steel cup 53 is formed with a conical surface 54 along the lower restricted end of the inner periphery thereof leading into a central aperture 55. The bottom of the cup 53 is beveled at the outer circumferenceto form an annular volume. 58 into which the mercury can ow after it radiates outwardly from the central aperture 55 under centrifugal force. The mercury then escapes from the annular volume 58 into the sump 30 through the indentations 52. Except for the construction pointed out above, this form of the switch is essentially the same as that shown in Figs. l and 2.

When a projectile containing the embodiment of the invention shown in Fig. 5 is fired, centrifugal force causes mercury to flow from the central aperture 55 between the roughened surface 51 and the at bottom surface 56 of the cup 53. The roughened surface 57 is held in close contact with the flat surface 56, and the mercury is retarded in flowing from the central aperture 55 radially outward through the field of minute craters between these surfaces into the annular volume 58. As described hereinbefore, the coarseness of the roughened surface 57 can be accurately reproduced, allowing control. ofthe time delay over a wide range and eliminating the necessity of one hundred percent inspection required on sintered metal restriction partitions.

In switches heretofore used, leakage of the mercury around the restriction partition into the sump 30 was not uncommon. Seepage of mercury around disks 16 and 17 of the modification shown in Fig. l is possible if the spinning operation which seals the disks 16 and 17 in place within the larger diameter section -15 is improperly performed. In the form of the invention shown in Fig. y5 any possibility of seepage of mercury around the restriction partition is completely eliminated. The only possible path for the conductive fluid out of the cup 53 is through the central aperture 55 and thence radially outward through the foraminous orifice formed between the flat bottom surface 56 and the roughened surface 57. Thus erratic. time-delay due to leakage of mercury around the restriction partition has been eliminated.

I'he preferred embodiment of the invention depicted in Fig. 7, which is only slightly modified from the form of the switch shown in Fig. 5, is particularly adapted to mass production. The disk 61 seated against the shoulder 13 is the same shape as the disk 5-1 utilized in the embodiment illustrated in Fig. 5, but neither at surface of the disk 61 is roughened. A thin stainless steel disk 62 of slightly smaller diameter than the inner diameter of the shell 11, and having the upper surface 63 thereof roughened as hereinbefore described, is positioned-between the disk 61 and the flat bottom surface 56 of the cup 53. The thin disk 62 can be conveniently fblanked from a narrow strip of 0.004 inch stainless steel which has been overblasted in an automatic operation wherein the stainless steel strip is fed continuously past the stream of grit to roughen the surface thereof. The use of thin disks 62 blanked from a roughened strip eliminates the necessity of Sandblasting the surfaces of individual small parts and thus greatly simplifies the roughening`operation. vThe thin disk 62 is assembled between the cup 53 and the disk 61 with the roughened surface 63 of the thin disk 62 adjacent the at bottom surface 56. Under centrifugal force mercury is caused to radiate outward from the orifice 55 between the field of minute craters formed between the abutting surfaces 56 and 63 and thence into the annular volume 58 to escape into thel sump 30 through the indentations 52. p Except for the variations pointed ont above, this form of -switch is essentially .the same as that depicted in Fig. 5. I.

While the embodiment of the present invention as herein described constitutes a preferred form, it is to be under-- stood that other forms might be adopted all coming Within the scope of theclaims which follow. `l-

Having; thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States: is:

l=. A centrifugal switch comprising a tubular shell closed at one end and having a reduced portion at said closedl end` formingY an annular shoulder intermediate its ends,` said shell alsoV having a pair of axially-spaced, inwardly-extending, annular lianges on the inner periphery near the open end thereof, a central contact member fitting snugly between said pair of flanges sealing the open end of said shell,` a hollow cylindrical member open at both ends fitting snugly within said shell between said shoulder and one of said flanges, mercury within said hollow member, means sealing the open end of said hollow. member at` said shoulder for retarding the ow of mercury from said hollow member into said reduced portion.. including a passageway consisting of the interstices between two ilat surfaces, at least one of which is sandblasted and each of which 'nas at least one mercury transmitting channel therethrough with the channels out of register with each other.

2. A centrifugal mercury switch in accordance with claim l inwhich said central contact member comprises av metallic central rod and av metallic flanged ring insulatedfrom said rod by4 glass, the metal of said rod and flanged ring having av low tendency to amalgamate with mercury and having substantially the same. thermal expansivity asl the glass that insulates the rod from the tiangcd ring.

3. A centrifugal switch comprising a tubular shell closed at one endand having a reduced portion at said closed end f fbrming an annular shoulder intermediate its ends, a central. contact member, means including1 a pair of annular flanges on the inner periphery near the open end of said shell. for sealing said contact member across the open end of' said shell and for insulating said contact from said shell, said shelly comprising. a second contact member, means dividingsaid shell into a chamber containing both mercury and said Contact member and a chamber remote from said contact member for retarding the flow oaf mercury into said remotecharnber including a pair of stainless steel members each formed with a at surface transverse of the axis of said shell with the surfaces held in close contact, each of said members having at least one mercury transmitting channel therethrough with the channels through the respective members out of register with each other, at least one of said fiat surfaces being sandblasted to allow a restrained flow of mercury through the restricted foraminous orifice provided between said contacting surfaces and through said channels, and means including saidvshoulder and one of said flanges for preventing seepage of mercury around said mercury retarding means into said remote chamber.

4i- A centrifugal mercury switch comprising a tubular metal shell closed at one end and having a reduced portion at said closed end forming an annular shoulder intermediate its'end, said shell having a pair of axially-spaced, inwardly-extending, annular flanges on the inner periphery near the open end thereof, a central Contact member fitting snugly between said pair of flanges sealing the open end of said shell and being insulated from said shell, mercuryretarding: means against said shoulder and having a'mercury containingchamber and a transverse foraminous passage defined by the interstices between a pair of contiguous surfaces at least one of which is sandblasted andiprovidedv with at least one aperture parallel to the-.axis of'. said. shell connecting said chamber and said p assageandv at least one outlet intersecting said passage, saidaperture, passage, and outlet permitting. a restrained flow of mercury from'- sai'd chamber, and means including said shoulder and one of said flanges for preventing seepageA of-"mercury from said chamber around said mercury `'rt-itarding.means into said reduced portions.

5r A'lcentrifuga'l mercury switch comprising a tubular shell closed at onefend andihavinganrannular shoulder in- 8 termediate its ends, a central contact member, means for sealing said contact member across the open end of said shell and insulating said contact member from said shell, the shell comprising a second contact member, time delay means dividing the switch into two compartments for retarding the flow of mercury from one compartment into the other comprising a flat-bottomed cup provided with a central `aperture through the bottom and a stainlesssteel disk formed with a plurality of circumferential indentations and having a sandblasted upper surface abutting against the bottom of said cup, mercury in said cup, and means including said shoulder and a cooperating flange on said shell for preventing seepage of mercury around said cup into the closed end of said shell and for holding the abutting surfaces of said cup and said disk in tight contact to form a restricted forarninous oritice between said surfaces which prevents passage of mercury radially outward from said central aperture to said circumferential indentations at atmospheric pressure under blows and impacts occurring during normal handling but allows passage of mercury under high pressure.

6. A centrifugal mercuryI switch comprising a tubular shell closed at one end and having an annular shoulder intermediate its ends, a central contact member, means for sealing said contact member across the open end of said shell and insulating said member from said shell, said shell comprising a second contact member, a flat circular plate seated against said shoulder and formed with a plurality of circumferential notches extending further toward the aXis of said shell than said annular shoulder, a flat-bottomed cup fitting snugly within said shell with the flat bottom thereof disposed transversely of and dividing said shell into two compartments, said cup being formed with a central aperture through the flat bottom thereof, a thin stainless steel disk disposed within said shell between said plate and the flat bottom of said cup and having a diameter smaller than the inner diameter of said shell, the surface of said disk abutting the fiat bottom of -said cup being sandblasted, and means for preventing seepage of mercury around said cup and for clamping said plate and said disk between said shoulder and said cup to provide a restricted foraminous orice between the sandblasted surface of said disk and the flat bottom of said cup capable of restraining the radial ow of mercury at atmospheric pressure between said surfaces from one compartment into the other under the blows and impacts occurring during normal handling but capable of passing mercury under high pressure.

7. In a centrifugal mercury switch, a tubular metal shell closed at one end and having an annular shoulder intermediate its ends, a central contact member, means for sealing said contact member across the open end of said shell and insulating said member from said shell, said shell comprising a second contact member, a stainless steel disk seated against said shoulder having ay fiat, sandblasted surface away from said shoulder, said disk being formed with a plurality of circumferential indentations extending further toward the Aairis of said shell than said annular shoulder, a fiat-bottomed cup fitting snugly within the shell with the iiat bottom thereof disposed transversely of and dividing the switch into two compartments, said cup being formed with a central aperture through the at bottom thereof, and means for preventing seepage of mercury around said cup and for clamping said disk between said cup and said shoulder to hold the sandblasted surface of said disk irrnly against the liat bottom of said cup in order to provide a restricted foraminous orice between the abutting surfaces capable of Vrestraining the radial llow of mercury at atmospheric pressure between said surfaces from one compartment into the other under the blows and impacts occurring during normal handling but capable of passing the mercury under high pressure.

8. A centrifugal mercury switch comprising a tubular metal shell closed at one end and having an annular shoulder intermediate its ends, a central contact member, means for sealing said contact member across the open end of saidyshell and insulating said Contact member from said shell, the shell comprising a second contact member, time delay means dividing the switch into two compartments for retarding the ow of mercury from one compartment into the other comprising a flat-bottomed cup provided with a central aperture through the bottom, a stainless steel disk formed with a plurality of circumferential indentations and having a sandblasted upper 10 1,883,720

surface abutting against the bottom of said cup, and means for holding said abutting surfaces in tight contact, mercury in said cup, and means for preventing seepage of mercury around said cup and disk into the chamber remote from said central contact, whereby a restricted foraminous orifice is formed between said abutting sur- 5 sage of mercury under high pressure.

References Cited in the le of this patent UNITED STATES PATENTS Grimes Oct. 18, 1932 2,298,908 Wentworth Oct. 13, 1942 2,396,283 Papst Mar. 12, 1946 2,436,602 Rickmeyer Feb. 24, 1948 2,449,862 Rickmeyer Sept. 21, 1948 15 2,509,527 Rickmeyer May 30, 1950 

